The present invention relates generally to optical communication systems, and more particularly, to a programmable optical vector modulator and method for use in coherent optical communication systems.
The assignee of the present invention designs coherent optical communication systems for use in communicating data over optical links. The optical links of these coherent optical communication systems have varying nonlinearity and varying optical signal to noise ratios.
Prior art optical signaling almost exclusively involves the use of incoherent on-off keying. The few coherent links that have been realized are binary. The present invention provides for a technique for generating arbitrary M-ary constellations. This allows multiple bits/symbol and predistortion of the constellation to compensate for some link non-idealities.
Prior art binary signaling cannot adapt to changing link conditions. The currently evolving optically switched, all-optical transcontinental network requires transmitters that can be reprogrammed from hour to hour to transmit on any link from 300 km to 10,000 km in length. The available link optical dynamic range and link nonlinearity varies widely as the link length varies.
It would be desirable to have a transmitter and method that can maximize the data rate according to changing link conditions. One way to do this is to have a transmitter and method that can generate arbitrary M-ary constellations that allow multiple bits/symbol and/or predistortion of the constellation to aid in compensating for link non-idealities.
For example, if on a given day a transmitter is required to transmit on a relatively long link with a low optical signal to noise, Quadrature Phase Shift Keying might be the only choice to achieve the required bit error rate. However, if the next day the transmitter is to transmit on a shorter, higher optical signal to noise ratio link it may be advantageous to switch to 8-ary Phase Shift Keying or a 16-ary format to transmit more data. Another reason to switch to a higher order format is if the traffic changes to a lower quality of service level and more errors can be tolerated; that is, error rate can be traded for more data throughput.
It is therefore an objective of the present invention to provide such an improved transmitter that can maximize the data transmitted according to changing link conditions. This invention comprises a programmable optical vector modulator and method for use in coherent optical communication systems.